Magnetic absolute encoders comprise a multipolar-magnetized rotor or magnetic drum, and a pair of magnetic detection elements. The multipolar-magnetized rotor or magnetic drum is attached in a coaxial configuration to the motor rotating shaft constituting the object of detection. The pair of magnetic detection elements are disposed in angular positions that are spatially separated by 90 degrees so that sine wave signals differing in phase by 90 degrees are output in association with the rotation of the motor rotating shaft. The outputs of the pair of magnetic detection elements are calculated, and the absolute angular position of the motor rotating shaft within one rotation (single-rotation absolute value) and number of rotations of the motor rotating shaft from a predetermined origin (multiple-rotation value) are detected.
Furthermore, the system is constructed so that when the main power supply is interrupted, a switch is made to an internal or external backup power supply so that power is supplied to various parts, thus preventing any interruption of the detection operation even in the case of an emergency such as an interruption of the power supply or the like Since the backup power supply has a limited capacity, the supply of power to various parts is performed intermittently, and the detection operation is backed up with low power consumption.
Here, a high-speed calculation capacity is required in the calculation circuit in order to perform high-precision detection at a high speed, whereas low power consumption is required during backup. High speed and low power consumption are in a trade-off relationship, and it is difficult to satisfy both of these requirements in a single detection system.
From this standpoint, in Patent Document 1, an encoder device is disclosed in which two detection systems are provided, and long-term backup is made possible by actuating a low-power-consumption detection system during backup
[Patent Document 1] JP-A 5-233064
However, in a conventional magnetic absolute encoder equipped with two detection systems, no consideration is given to countermeasures against detection errors occurring in the main detection system which performs high-speed, high-precision detection.